The formation of various integrated circuit (IC) structures on a wafer often relies on lithographic processes, sometimes referred to as photolithography, or simply lithography. As is well known, lithographic processes can be used to transfer a pattern of a photomask (also referred to herein as a mask or a reticle) to a wafer.
For instance, patterns can be formed from a photo resist layer disposed on the wafer by passing light energy through a mask having an arrangement to image the desired pattern onto the photo resist layer. As a result, the pattern is transferred to the photo resist layer. After a development cycle, the photo resist material can be selectively removed to expose an underlying layer (e.g., a semiconductor layer, a metal or metal containing layer, a dielectric layer, a hard mask layer, etc.). Portions of the photo resist layer that are not removed serve to protect the underlying layer during further processing of the wafer (e.g., etching exposed portions of the underlying layer, implanting ions into the wafer, etc.). Thereafter, the remaining portions of the photo resist layer can be removed.
There is a pervasive trend in the art of IC fabrication to increase the density with which various structures are arranged. For example, feature size, line width, and the separation between features and lines are becoming increasingly smaller. For example, nodes with a critical dimension (CD) of about 45 nanometers (nm) to about 65 nm have been proposed.
To assist in forming structures with a small CD, various lithographic techniques and optical parameters have been employed. For instance, use of a relatively small illumination wavelength (e.g., 193 nm) has been proposed in conjunction with a relatively high numerical aperture (NA), such as about 0.6 to about 0.95 or even higher. Unfortunately, this combination of optical parameters can lead to a reduction in contrast of the image incident on the photo resist.
Accordingly, there exists a need in the art for improved techniques and wafer assemblies for imaging a pattern onto a layer of photo resist using lithography.